'object' option not available in Object Solver panel
I do not know what tutorial you are following, but I think you might have missed something. The modifier is on the cube, but the cube is the object you are trying to track to is what it sounds like. You can't use the object which is being modified as the target object in any field, in any modifier. The tutor might have duplicated the object, or done something without proper narration, but as far as I know the target object can never be the object being modified.